Conventional techniques for handling semiconductor wafers (such as silicon wafers) during processing, and in particular for transferring wafers between processing tools, typically involve standard-sized cassettes which hold a plurality of wafers at a time; e.g., about 14 wafers to a cassette. The wafers are typically supported by their edges by “fins” protruding inward from the sides of the cassettes. Handling chucks or “forks” associated with robotic and/or automatic equipment are used to remove wafers from the cassettes one at a time by supporting the wafer from the bottom, inboard of the cassette fins. The forks bring the wafers to and from processing equipment, and place the wafers into the cassettes.
The above-described handling techniques were developed for wafers that are substantially rigid, such as wafers having a thickness of about 250 μm or greater. However, recently developed device packaging schemes; e.g., “chip-sized packages” used in miniaturized products such as cell phones, are significantly smaller than conventional packages, and require thinner semiconductor devices. Therefore, thin silicon wafers are currently being produced, having a thickness of about 150 μm or less. These thin wafers, which can have a diameter of 8 inches (200 mm) and a thickness of only 100 μm, typically start processing being much thicker (e.g. about 500 μm thick) and are ground from their back side (“back ground”) to a desired final thickness.
Conventional wafer backgrinding is typically performed in many steps using an integrated, dedicated progressive grinder/polisher. Such machinery is very expensive, and is only as fast as its slowest component. Therefore, backgrinding is a “bottleneck” in wafer processing, reducing throughput and raising production costs.
A further disadvantage of conventional processes is that thin semiconductor wafers tend to warp and sag after backgrinding, due to backgrinding induced stresses and other mechanical stresses. When placed in conventional wafer cassettes, the sagging thin wafers touch each other. Moreover, when a fork attempts to remove a sagging and/or warped wafer from the cassette, the fork can easily hit the wafer and scratch or otherwise damage it, since the wafer is not within the “reference window” of a predetermined reference plane. In other words, the wafer is not where the fork expects it to be, because the warped and/or sagging wafer is not located in a predictable “work plane”. Thus, the fork cannot safely handle the thin wafer.
There exists a need for a cost-effective, less time-consuming technique for thinning wafers. There also exists a need for a method and apparatus for handling thin wafers without damaging them, thereby reducing manufacturing costs and increasing yield.